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BISMARCK, North Dakota — A truck driver who lost control of a semitrailer carrying missile parts was distracted by a "large insect" that flew in a window and landed on the driver's back, the military said in a report released Friday.

The Air Force said the truck from Minot Air Force Base's 91st Missile Wing, which overturned Aug. 31 on a gravel road in northwest North Dakota, was carrying rocket engine parts for intercontinental ballistic missiles but no nuclear material. The shipment also contained two tanks of liquid rocket fuel, but never was in danger of exploding or leaking, the military said.

It was the second crash of a base vehicle in just more than a year. The Air Force spent about $5.6 million last year to recover an unarmed booster rocket for an intercontinental ballistic missile after the truck carrying it overturned in July 2008. The military blamed "driver and safety observer error" for that accident and said the public never was in danger.

The Air Force report released Friday also blamed the latest accident on driver error, saying "the driver became distracted ... when a large insect flew into the driver's open window and landed on the driver's back."

The truck drifted while the driver tried to remove the insect and tipped over in a ditch, according to the report.

"The investigation doesn't list what kind of bug it was," said Andy Roake, a civilian spokesman for the Air Force at Peterson Air Force Base in Colorado Springs, Colorado. "I don't know if the driver really knew."

The driver and a passenger, whose identities were not released, were not injured, the military said.

The Air Force recovered the semitrailer and its cargo Sept. 4. Roake said the cost of the crash and its cleanup still was being tallied.

The Minot base, home to about 4,800 active-duty military personnel, is the command center for about 150 Minuteman III missiles — sunk in hardened silos in north central and northwestern North Dakota — and is one of the nation's two B-52 bomber bases.